public void ValidateInput_Function_Test()
{
var calculator = new Calculator1();
Assert.AreEqual(-1, calculator.ValidateInput_Function("three"));
Assert.AreEqual(-1, calculator.ValidateInput_Function("87654"));
Assert.AreEqual(11, calculator.ValidateInput_Function("11"));
Assert.AreEqual(-1, calculator.ValidateInput_Function("0"));
Assert.AreEqual(-1, calculator.ValidateInput_Function("1.5"));
}